1. Field of the Invention
The present invention broadly relates to measuring the amount of extent of halation due to surface deterioration in transparencies, such as aircraft windscreens made of plastic or the like, and, more particularly, is concerned with a method and apparatus for providing a record of the halation measurement in terms of human visual perception, i.e., as a person would see the tested region of the transparency were he to visually inspect the transparency.
2. Description of the Prior art.
The requirement for effective birdstrike protection in todays's high-performance aircraft has caused a transition from glass canopies and windscreens to laminated or monolithic plastic transparencies. Plastic aircraft transparencies have a life cycle which appears to be limitedby the surface qualities of the transparency. Since the hardness of the plastic surface is less than that of glass, the plastic transparencies are much more susceptible to environmental surface damage than the glass transparencies which they replace.
One of the most significant optical changes that occurs during the life cycle of the plastic transparency is an increase in haze or halation due to the surface deterioration of the transparency. As halation increases, light appears to "spread" from its source, causing a disabling glare or significant reduction in contrast of objects seen through the transparency. This disabling glare can lead to flight safety problems as the pilot's view of the external world is restricted.
Halation, the spreading of light beyond its proper boundaries due to internal reflections from scratches, is evident whenever a bright light source appears in the field of view of the pilot. Some of the light scatter occurs in the pilot's eye, and is often perceived as a dim halo or ring of light encircling the light source as it is viewed at night. Another source of halation is the "volume haze" inherent in most aircraft transparencies. A third, and probably most significant source of halation is the accumulation of scratches on the surface of the transparencies. The effect of aging on a transparency is due to this source of halation. These scratches tend to cause light to be more evenly distributed over the surface of the transparency, resulting in disabling glare or reduction of contrast between a target and its background. SInce the human eye is a contrast detector, and since target acquisition and tracking are based on maintaining sufficient contrast for the visual system to be operative, reduction in contrast will reduce the ability of the pilot to see objects through the transparency. If the disabling glare is sufficiently widespread, portions of the transparency may become unusable which results in effectively reducing the visual field of the pilot. The loss of visual field can lead to flight safety problems and loss of combat effectiveness.
Current methods of measuring haze are limited to measuring volume haze only. Even then, the measurement methods can only be performed on small, flat transparency samples held in a specific relationship to the measuring device. The methods are neither suitable for the measurement of surface scratch induced haze, nor do they depict the appearance or the effect of halation on the pilot's visual abilities.
These methods of measuring haze commonly use an integrating sphere type hazemeter instrument. (One such instrument is called the Gardner Hazemeter.) After calibration of the instrument, a small sample of the transparent material is placed in front of the sphere aperture, and a reading is taken of all of the light impingent on the inner surface of the sphere. A second reading is taken while the non-diffused light is allowed to exit the sphere or fall into a light trap. The haze measurement, in percent, is then calculated from a ratio of the two readings. Although this is an effective method of comparing the "haze" of various transparent materials, it can not and does not accurately predict the visual effect upon the observer.
The above-described measurement method has significant disadvantages for field use. First, the sample to be tested must be held perpendicular to the aperture, so that all readings are taken along an axis which is normal to the surface of the transparency. Next, the sample must contain no surface scratches. If the sample does contain scratches, they should be removed by immersing the sample in a liquid of equal index of refraction. Finally, the surface of the sample must be flat and in contact with the aperture of the test instrument.
Some of these disadvantages have been overcome by several transparency manufacturers and the Air Force Aerospace Medical Research Lab through modification of their Gardner Hazemeters to accept large transparencies. These modifications have allowed haze readings to be taken over virtually the entire surface of a slightly curved transparency, and over a lesser area of a significantly curved transparency. However, measurement errors may be exacerbated by the remaining disadvantages mentioned above. In any case, modification of hazemeters has not allowed halation measurements to be taken under field conditions, while the transparency remained on the aircraft. Furthermore, no real relationship had been shown between haze as measured by the ASTM or FTM methods and visual performance. Finally, the instrumentation is bulky and for that reason too is not suited for use under field conditions.
In summary, at the present time there is no apparatus available to objectively measure the effect of halation upon vision, nor reliably measure the extent of halation in aircraft windscreens, canopies or any large or curved transparencies. Thus, there is no objective metric to indicate the extent of visual hazards due to halation, nor to indicate a proper time for removal and replacement of the transparency.